Baluns are used to interface balanced systems to unbalanced systems, and to transition electrical energy therebetween. In fact, the word “balun” is derived from the “bal” of balanced and the “un” of unbalanced. Many antennas interface with a feedline having different balance characteristics. A dipole antenna is a common example where an inherently balanced antenna often uses unbalanced transmission lines for the feed line. As a result, when using a dipole antenna or other balanced antenna system, baluns are often added to transition balanced or nearly balanced terminal voltages of an antenna to unbalanced voltages of a feedline while maintaining equal and opposite currents at any instant of time in and out of the interface. The balun also transitions a balanced signal voltage transmitted or received by the antenna from or to the unbalanced voltage of a coaxial feed line.
Requirements for baluns and balanced to unbalanced interfaces are not limited or restricted to dipole antennas, antennas of other types, or transmission lines, but may also include transmission line to transmission line interfaces, generators, transmitters, receivers, and other devices that absorb, convey, or supply time-varying currents and voltages for transmission or reception of radio frequency energy. The more difficult situation is substantially improving wide frequency range radio communications systems by improving transitions between or among antennas and feedlines to transmitters and transmitter/receiver combinations, especially when the balun is used in a variety of unpredictable system constructions and operating conditions at modest-to-high power levels over wide frequency ranges. Higher power operation over wide frequency ranges with a wide variety of end-use conditions requires special care and consideration not available through traditional balun construction.
Although some baluns transform impedances when transitioning between balanced to unbalanced systems, the main function of a balun is to provide proper isolation of current paths and voltage differences between balanced and unbalanced voltage systems. As one example, the need for a balun, and the isolation of paths provided by the balun, is seen when the balanced voltages of dipole antenna feedpoints are attached to unbalanced voltages of a coaxial feed line. While this example is of a dipole antenna, balance and unbalance also applies to other antenna systems and feedlines, which always must be someplace between being perfectly balanced and perfectly unbalanced in voltages while generally requiring exactly equal and opposing currents for optimum performance or satisfactory operation. In this example, a first dipole arm and a second dipole arm form a balanced or nearly balanced voltage load for the transmission line. The first dipole arm or balanced load terminal is attached directly to the inner conductor of the coaxial cable and the second dipole arm is attached directly to the outer conductor of the coaxial cable.
When any balanced voltage or less than perfectly unbalanced voltage antenna system is operating without a balun and connected to an unbalanced voltage transmission line, a first current flows in one direction at one instant of time through the first dipole arm and the inner conductor. At the same instant of time a second opposite direction current flows oppositely along the inside wall of the coaxial outer conductor and a portion reaches and flows into the second dipole arm. However, a third unwanted current develops where the second dipole arm is attached to the outer conductor of the unbalanced feedline. In this dipole example, an electrical voltage appears at the attachment point for the second current, and this voltage causes a third current and unwanted voltage to be created along the outer surface (or shield) of the coaxial cable. That is, the desired transmission line power is divided into two power components. The first power or energy component flows to or from the desired place known as the antenna, and a second unwanted power component appears from an undesired third current and voltage along the outside of the shield. As a result, the desired power is effectively divided into an unwanted and harmful power caused by unwanted current and voltage in an undesired place.
The creation of the third unwanted current results in unwanted and undesired radiation or reception from the feed line, and undesired unequal currents in the dipole arms. Such radiation and unequal currents consume power from the energy transferred between the antenna and the receiver, generator, or transmitter system, and, therefore, decrease efficiency and performance of the entire system. However, the magnitude of the disturbance in voltages and undesired third current depends on the impedance of the outside surface of the coaxial cable and the voltage driving that unwanted current. For example, if the impedance of the surface of the coaxial cable, antenna, other transmission line, or load is very high, then the amount of electrical current generated at the above-described transition point is low, and, therefore, the amount of useful and wanted electrical power converted into an undesired and harmful power is low. Consequently, when the impedance on the outside surface of a coaxial cable is high, the power is not divided, and the third unwanted current is effectively eliminated. The same is true for a balanced transmission line connected to an unbalanced source, a radio transmitter being a source; or an unbalanced load, an antenna or other circuit being a common load.
Therefore, if the impedance of the outside surface of the coaxial cable can be increased, then the radiation from the feed line and the unequal currents and voltages in the dipole arms due to the third current can be eliminated as a problem. To that end, the purpose of the balun is to increase the impedance along the outside surface of the transmission line, restricting unwanted diversion of useful power to useless or harmful power at the transition point.
Three configurations of baluns have traditionally been used in high-power transmitting antennas used over wide frequency ranges. The first configuration was popularized by Walter Maxwell, and consists of a plurality of ferrite beads strung over the feed line. The second configuration was popularized by Jerry Sevik, and consists of a transmission line wound through and around a magnetically soft iron toroid. The third configuration, the air-core balun, is a basic coil or winding of wire wound in a hollow circle with or without a supporting form.
However, because the desired impedance increase provided by the core-type baluns depends almost totally on the amount of transmission line passing through the core of the baluns, the Maxwell and Sevik baluns have inherent limitations. The air-core balun is also severely limited, because good performance requires a large and bulky construction, which restricts bandwidth and wastes space. Moreover, the Maxwell balun wastes core material because the transmission line passes only once through the holes provided in the ferrite beads. One unit length of bead only provides one unit increase of impedance. Furthermore, the Sevik balun concept, while making good use of the core, wastes transmission line length because for every wind around the toroid, the transmission line passes only once through the hole of the toroid. Most of the transmission line is outside the desired magnetic field concentrations of the core. As a result, there is a need for a balun that minimizes the relative amount of transmission line and core material necessary, and at the same time greatly maximizes the amount of desired isolating impedance on the outside ends of the balun while maintaining very wide bandwidth.